EIP: Rates, Ratios, Risks - Week 11

Question 1

Do cross-sectional studies imply cause and effect?

Answer 1

No

Question 2

Can cross-sectional studies tell us what factors are causing a health problem?

Answer 2

No

Question 3

What can cross-sectional studies describe?

Answer 3

‘state of play’

Question 4

In comparison to cross-sectional studies, how would you describe a cohort study?

Answer 4

longitudinal, unlike cross-sectional

Question 5

What type of study shows us a ‘snapshot’: cross-sectional or cohort?

Answer 5

cross-sectional

Question 6

What is an odds ratio?

Answer 6

A comparison of the odds of something in 2 different groups
- it gives us the odds of something being true in one group compared to another

Question 7

Are odds the same as probabilities?

Answer 7

No. Therefore not the same as relative risk

Question 8

How frequently are odds and risks similar?

Answer 8

often

Question 9

How would you find the relative risk of something?

Answer 9

by finding the proportion in a population (it’s a probability so divide by total and there you go)

Question 10

Let’s assume you find 10 myopes in a sample of 45. What are the ODDS of any one individual being a myope?

Answer 10

10:35 (or 10 myopes to 35 non-myopes, or 0.29 to one)

Question 11

Let’s assume you find 10 myopes in a sample of 45. What is the RISK of any one individual being a myope?

Answer 11

10/45 = 0.22 (a probability)

Question 12

Which is more important to use: Odds Ratio or Relative Risk? (2)

Answer 12

Depends on the study design
- odds ratio should be used in cross-sectional studies

Question 13

Are case control studies retrospective or prospective?

Answer 13

retrospective

• you start with the state under investigation already present and go back to try and identify the cause

Question 14

Can case control studies answer questions about causation?

Answer 14

No. They are too prone to bias

Question 15

Are case control studies useful for hypothesis generation?

Answer 15

yes

Question 16

Define Control Event Rate (CER)

Answer 16

The probability of developing an outcome for the control group
CER = c/(c+d)

Question 17

Define Experimental Event Rate (EER)

Answer 17

The probability of developing an outcome for the experimental (treatment) group
EER = a/(a+b)

Question 18

Define Relative Risk (RR)

Answer 18

The ratio of the probability of developing an outcome among those receiving the treatment of interest or exposed to a risk factor, compared with the probability of developing the outcome if the intervention or risk factor is present

RR = EER/CER

Question 19

Define Relative Risk Reduction (RRR)

Answer 19

The extent to which a treatment reduces a risk, in comparison with patients not receiving the treatment of interest

RRR = (CER - EER)/CER

Question 20

Define Absolute Risk Reduction (ARR)

Answer 20

The difference in the rates of adverse events between experimental and control groups

ARR = CER - EER

Question 21

Define Number needed to treat (NNT)

Answer 21

The number of patients who must be exposed to an intervention before the outcome of interest occurred (e.g. the number of patients needed to treat to prevent one adverse outcomes)

NNT = 1/ARR